1. Field of the Invention
The present invention relates to a coupling device, and an image forming apparatus.
2. Description of the Related Art
A typical electrophotogrphic image forming apparatus forms an image by developing an electrostatic latent image formed on an image carrier using a developer and transferring the latent image to a recording material. A typical process cartridge houses, in its housing that is detachable with respect to an apparatus body of an image forming apparatus, at least one of a charging unit, a developing unit, and a cleaning unit that are arranged at a circumference of a drum-shaped photoconductor, and an image carrier.
FIG. 29 is a schematic of a state before a process cartridge 201 is mounted on an image forming apparatus body, and FIG. 30 is a schematic of a state where the process cartridge 201 is mounted.
The process cartridge 201 includes a photoconductor 202, and a developing unit 205 as a driven unit. About the horizontal direction in these figures, the left side is the front side (the near side) of the image forming apparatus, and the right side is the rear side (the far side) of the image forming apparatus.
A drum shaft hole (not shown) is provided on a rear flange 202b of the photoconductor 202. A concave gear 221 having a conical pitch surface with the drum shaft hole as its center is provided on the outer surface of the rear flange 202b. A drum shaft hole 202e is provided on the center of a front flange 202c. The photoconductor 202 is supported by supporting units (not shown) provided on a rear surface plate 211, and a front surface plate 218 provided on both sides of the photoconductor 202 in the shaft direction. The developing unit 205 includes a developing roller 205g positioned by the rear surface plate 211, and the front surface plate 218. The developing unit 205 includes a developing gear 258, an idler shaft 259 provided on the rear surface plate 211, a driven gear 260 rotatably provided on the idler shaft 259.
A cylindrical mating frame 270 that mates with a shaft bearing 215 fixed to the drum shaft is provided on the rear surface plate 211, and a shaft bearing 271 is attached to the front surface plate 218.
The apparatus body includes a front plate 225, and a rear plate 291. A retaining plate 289 is fixed to the rear plate 291, and a drive motor 281 is attached to the retaining plate 289. The rear plate 291 rotatably supports a drum shaft 202a penetrating the photoconductor 202 in the shaft direction through a shaft bearing 290. A coupling unit 293 such as a coupling couples the drum shaft 202a linearly with a motor shaft 281a of the drive motor 281. A first pulley 286, a convex gear 220 having a conical pitch surface, and the shaft bearing 215 are fixed to the drum shaft 202a. 
The rear plate 291, and the retaining plate 289 rotatably support a drive shaft 282 for rotation-driving the developing roller 205g through shaft bearings 284a, 284b. A second pulley 283 is fixed to the drive shaft 282, and a timing belt 285 is wrapped around the second pulley 283, and the first pulley 286. A drive gear 262 is fixed to the front end of the drive shaft 282. A shaft bearing 226 that supports the front end of the drum shaft 202a is provided on the front plate 225 of the apparatus body.
When the process cartridge 201 is mounted while the front plate 225 of the apparatus body is open, the drum shaft 202a penetrates the photoconductor 202 as shown in FIG. 30, and the concave gear 221, and the convex gear 220 mate with each other. Simultaneously, the cylindrical mating frame 270 mates with the shaft bearing 215 on the drum shaft 202a, and the process cartridge 201 is positioned to the apparatus body. On the developing unit 205 side, the driven gear 260, and the drive gear 262 mesh with each other.
In the image forming apparatus shown in FIGS. 29 and 30, the idler shaft 259 to which the driven gear 260 for rotation-driving a rotating body such as the developing roller 205g of the developing unit 205 arranged in the circumference of the photoconductor 202 is fixed to the rear surface plate 211 of the process cartridge 201. The drive shaft 282 to which the drive gear 262 that meshes with the driven gear 260 is fixed is supported: on the apparatus body side. Accordingly, when the process cartridge 201 is positioned to the apparatus body with the drum shaft 202a as a reference, the distance between the shaft center of the idler shaft 259 and the shaft center of the drive shaft 282 may vary beyond a predetermined range due to the accumulation of tolerance. When the distance of the shaft centers vary beyond a predetermined range, vibration is generated when the driven gear 260 and the drive gear 262 mesh with each other to transmit drive force. This vibration is transmitted to the photoconductor 202, and causes image degradation such as banding.
Japanese Patent Application Laid-open No. 2004-45603 discloses a coupling unit capable of transmitting drive force even when the shaft centers of the driven shaft and the drive shaft misalign with each other, and that prevents occurrence of vibration at the time of transmitting the drive force.
FIGS. 31A to 31C are schematic diagrams of a coupling 316 disclosed in Japanese Patent Application Laid-open No. 2004-45603. FIG. 31A is a schematic of a state before a drive shaft 320 and a driven shaft 315 are coupled with each other, and FIG. 31B is a schematic of a state after the drive shaft 320 and the driven shaft 315 are coupled with each other. FIG. 31C is a schematic of the coupling 316 seen from the drive shaft 320 side.
As shown in the figures, the coupling 316 includes a tubular insertion part 319 coupled with the driven shaft 315, and a shaft insertion part 318 to which the drive shaft 320 is inserted. A long guide hole W is provided on the tubular insertion part 319. The driven shaft 315 is inserted into the centrum of the tubular insertion part 319. The guide hole W is superposed on a through-hole (not shown) provided near the drive side tip of the driven shaft 315, and a slide pin 331 is inserted through the guide hole W, and is press-fitted to the through hole; thereby, the coupling 316 is attached to the driven shaft 315.
A spring bearing 332 is fixed to the driven shaft 315, and a coil spring 317 is disposed between the spring bearing 332, and the coupling 316; thereby, the coupling 316 is biased to the drive shaft side.
An inner diameter a of the tubular insertion part 319 is larger than a diameter b of the driven shaft 315, and the coupling 316 is attached to the driven shaft 315 with clearance Q therebetween. By providing the clearance Q between the driven shaft 315 and the tubular insertion part 319 of the coupling 316 in this way, the coupling 316 can oscillate about the slide pin 331.
As shown in FIG. 31C, two catch portions V protruding toward the shaft center are provided on the shaft insertion part 318 of the coupling 316. A through-hole (not shown) is provided near the tip of the drive shaft 320 on the driven shaft side, and a drive pin 330 is press-fitted in the through hole.
As shown in FIG. 31B, when the drive shaft 320 is inserted into the shaft insertion part 318 of the coupling 316 while the shaft center of the drive shaft 320, and the shaft center of the driven shaft 315 misalign with each other, the coupling 316 rotates about the slide pin 331, and inclines relative to the driven shaft 315. Because the coupling 316 inclines in this way, the drive pin 330 press-fitted to the drive shaft 320 can be inserted into the shaft insertion part 318. As a result, even when the shaft center of the drive shaft 320 and the shaft center of the driven shaft 315 misalign with each other, the drive pin 330 engages with a side surface Va of the catch portions V, and the drive force is properly transmitted to the driven shaft 315. Vibration is never generated at the time of transmitting the drive force. Accordingly, image degradation such as banding can be suppressed.
However, with the coupling 316 disclosed in Japanese Patent Application Laid-open No. 2004-45603, when the shaft centers of the driven shaft 315 and the drive shaft 320 misalign with each other, as shown in FIG. 32A, only one of the protrusions of the drive pin 330 that protrude from the drive shaft 320 apart from each other at an interval of 180 degrees engages with the catch portion V of the coupling 316. At the time of rotation, the protrusion of the drive pin 330 that engages with the catch portion V of the coupling 316 switches from one to another as shown in FIG. 32B. When the protrusion of the drive pin 330 that engages with the catch portion V of the coupling 316 has switched, the position at which the catch portion V of the coupling 316 engages with the drive pin 330 changes from the base of the protrusion to the tip of the protrusion of the drive pin 330. As the rotation continues, the position at which the catch portion V of the coupling 316 engages with the drive pin 330 moves to the shaft center side (the base side) of the drive shaft 320. The circumferential speed of the tip of the protrusion of the drive pin 330 is faster than that of the drive shaft side of the protrusion of the drive pin 330. Therefore, the rotation speed transmitted to the coupling 316 when the tip of the protrusion of the drive pin 330 engages with the catch portion V of the coupling 316 shown in FIG. 32B is faster than the rotation speed transmitted to the coupling 316 when the drive shaft side of the protrusion of the drive pin 330 engages with the catch portion V of the coupling 316 as shown in FIG. 32A. As a result, rotational irregularity occurs in the developing roller 205g in the coupling of Japanese Patent Application Laid-open No. 2004-45603. Rotational irregularity of the developing roller 205g causes toner concentration irregularities resulting in image degradation. Concretely, when the rotation speed of the developing roller 205g is slow, the amount of developer that adheres to the photoconductor 202 is small, and when the rotation speed of the developing roller 205g is fast, the amount of developer that adheres to the photoconductor 202 is large.